Semiconductor capacitors are fundamental components for integrated circuits. A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed under the influence of a bias voltage. A variable capacitor is often used in inductor-capacitor (LC) circuits to set the resonance frequency of an oscillator, or as a variable reactance, e.g., for impedance matching in antenna tuners. One example type of variable capacitor is referred to as a transcap (TC) device, which is a metal-oxide semiconductor (MOS) based variable capacitor having at least three terminals, one of which is used to modulate the capacitance across two terminals of the TC device.
A voltage-controlled oscillator (VCO) is an example circuit that may use a varactor in which the size of a depletion region formed in a p-n junction diode is varied by changing a bias voltage to alter the junction capacitance. Any junction diode exhibits this effect (including p-n junctions in transistors), but devices used as variable capacitance diodes are designed with a large junction area and a doping profile specifically chosen to improve the device performance, such as quality factor and tuning range.